Spain arrested 11 people on suspicion of being part of a militant Islamist cell planning an attack in the northeastern region of Catalonia, a Catalan police spokeswoman said.

About 360 police took part in early morning raids at 17 properties, part of a year-long investigation that also has included the arrests of three Spaniards attempting to enter Syria from Bulgaria in December, the spokeswoman said.

"The arrested are assumed to be implicated in diverse crimes related to jihadi terrorism, especially linked to Islamic State," the police said in a statement.

Spain has arrested more than 30 people for suspected militant activity this year and is making efforts to stop the radicalization of young Muslims and prevent them from joining armed groups in Syria or Iraq or attacking targets at home.